Multiscale Modeling and Experimental Study of Defect Formation during Aluminum Alloy Solidification

金属合金在铸造过程中发生的凝固（从液体到固体的转变）在纳米尺度上决定了所得到的材料的结构及其力学性能。在铸造过程中的凝固过程中，液态金属表面与环境发生反应，产生金属氧化物，称为氧化膜。这些薄膜可以导致材料中不同类型的缺陷，特别是在铝合金中，因为它们的高氧化率。氧化膜在降低铝铸件的质量和可靠性方面起着主要作用，并且可以占铸件总有效问题的80%。该奖项支持研究铝合金凝固过程中氧化膜形成和演变的基础研究。本研究结果将为控制和减少铝铸件缺陷提供实用的建议。这项研究应用于汽车、航空航天和其他需要轻质、高强度和抗疲劳金属合金的行业。该项目还将有助于制造更轻、更节能的汽车。氧化膜现在被认为是造成微裂纹、微孔隙和其他疾病的主要原因，这些疾病大大削弱了铸件的机械性能。虽然有实验证据与这一假设相一致，但双歧膜在产生缺陷中的作用和缺陷产生的机制主要是猜测，因为没有直接观察是可能的，双歧膜的机械性能是未知的，并且双歧膜的物理演化和与结晶枝晶在凝固过程中的相互作用尚不清楚。研究小组将进行原子模拟，以确定氧化膜的高温机械性能。这些结果将被转移到一个新的多相场模型中，首次模拟双膜与凝固枝晶之间的相互作用，并跟踪双膜在凝固过程中的演变和变形。宏观尺度的热流体分析将用于确定相场模型微观尺度区域的流体流动和热边界条件。将进行铸造和表征实验来验证模型预测。

Solidification (liquid to solid transition) occurring during the casting of metallic alloys plays an important role in determining the structures of the resulting materials at the nanometer scale, as well as their mechanical properties. During solidification in a casting process, the liquid metal surface reacts with the environment, resulting in generation of metallic oxides, which are called oxide bifilms. These bifilms can lead to different types of defects in materials, especially in aluminum alloys because of their high rate of oxidation. Oxide bifilms play a major role in reducing the quality and reliability of aluminum castings and can account for as much as 80% of the total effective problems in castings. This award supports fundamental research to study oxide bifilm formation and evolution during solidification of aluminum alloys. The outcomes of this research will enable practical recommendations for controlling and reducing defects in aluminum casting. This research has applications in the automobile, aerospace, and other industries that demand lightweight, high strength and fatigue-resistant metallic alloys. This project will also contribute to the manufacture of lighter and more energy-efficient vehicles.Oxide bifilms are now believed to be the main cause of micro-cracks, microporosity, and other ailments that greatly weaken the mechanical properties of cast parts. While there is experimental evidence that is consistent with this hypothesis, the role of bifilms in producing defects and the mechanisms of defect creation are mostly conjecture, because no direct observation has been possible, the mechanical properties of bifilms are unknown, and the physics of bifilm evolution and interaction with crystalline dendrites during solidification is not understood. The research team will perform atomistic simulations to determine the high temperature mechanical properties of oxide bifilms. These results will be transferred to a new multi-phase-field model to simulate, for the first time, the interactions between bifilms and solidifying dendrites and to track the evolution and deformation of bifilms during solidification. Macro-scale thermal-fluid analysis will be performed to determine fluid flow and thermal boundary conditions for the micro-scale regions of the phase-field model. Casting and characterization experiments will be performed to validate the model predictions.